Anodically produced oxide coatings are frequently applied to aluminum surfaces for the purpose of protection against corrosion. These oxide coatings protect the aluminum surfaces against the influences of weather and other corrosive media. Furthermore, the anodic oxide coatings are also applied in order to obtain a harder surface and thus to increase the wear resistance of the aluminum. A special benefit from these oxide coatings lies in the decorative effects which may be obtained by virtue of the inherent color of the oxide coatings or by virtue of the fact that, in some cases, they can be readily colored.
A number of methods of applying anodic oxide coatings to aluminum are known. By way of example, the oxide coatings can be produced by direct current in solutions of sulphuric acid (direct current/sulphuric acid method). These coatings can be subsequently colored by immersion in solutions of a suitable dye or by treatment with alternating current in an electrolyte containing metallic salts. However, solutions of organic acids, particularly sulphophthalic acid or sulphanilic acid or, alternatively, these acids mixed with sulphuric acid, are also frequently used for the purpose of applying the oxide coatings. The last-mentioned methods are known as color anodization methods.
However, these anodically applied oxide coatings do not fulfil all requirements with respect to protection against corrosion, since they have a porous structure. For this reason, it is necessary to after-seal the oxide coatings. This after-sealing is frequently accomplished by means of hot or boiling water. The pores are thereby closed and thus the anti-corrosive effect is considerably increased.
However, in addition to closing the pores, the after-sealing of anodically applied oxide coatings also results in the formation of a more or less thick velvety layer, the so-called sealing layer, on the entire surface. This sealing layer comprises hydrated aluminum oxide and is not resistant to handling, thus adversly affecting the decorative effect of the coating. Furthermore, this sealing layer reduces the adhesive strength when aluminum members having such a layer are glued together and this layer promotes subsequent soiling and corrosion as a result of its enlarged effective surface. For these reasons, it was previously necessary to mechanically remove the layer by hand or by chemical methods.
It is already known to remove this sealing layer from sealed surfaces by after-treatment with mineral acid. In this method, a further treatment step is therefore necessary and, moreover, very careful after-treatment with mineral acid is required in order to prevent damage to the coating. Furthermore, in order to avoid sealing layers, the prior art includes after-sealing with solutions containing nickel acetate and lignin sulphonate. This method of operation is disadvantageous in that, inter alia, the oxide coatings obtained turn yellow under the influence of light. Finally, methods have also been described in which the formation of sealing layers is prevented by hot water sealing with the addition of specific polyacrylates or specific dextrins. These methods have proved to be satisfactory. However, drying residues can remain behind in many cases, particularly when the method is not carried out carefully. These residues are undesirable but they can be readily removed by after-rinsing.
It has also been proposed to use small quantities of hydroxycarboxylic acids, such as citric acid, tartaric acid, gallic acid and various phosphonic acids for the purpose of preventing the formation of sealing layers. However, when these substances are used, it has transpired that difficulties can arise with the over-metering of the effective substance, particularly in large baths which are badly circulated. Thus, it is not always a simple matter to adhere to the range of concentrations in which, on the one hand, the sealing layer is absolutely and reliably prevented and, on the other hand, the result of the short-time tests are not negatively influenced.